1. Field of the Invention
The present invention relates to an electronic device, more particularly to an air-tightly sealed electronic device having mounted thereon a microelectromechanical system element or other functional elements, and a method of production of the same. The present invention also relates to an improved method of securing a lid useful for production of such an electronic device.
2. Description of Related Art
In recent years, functional components, functional elements, functional devices and the like based on microelectromechanical systems (MEMS) (hereinafter referred to overall as “MEMS elements”) have come into attention. MEMS elements are extremely small in size, yet exhibit complicated and sophisticated functions. An MEMS element usually comprises a substrate having formed integrally with its active surface a mechanically operating micromachine structure (hereinafter referred to as a “micromachine component”), an electronic component for operating the micromachine component, etc. A micromachine component generally can be produced by micromachining. Specific examples of micromachine components include for example relays, sensors, etc. Electronic devices carrying such micromachine components can be advantageously utilized in the field of information communications, automobiles, household electrical apparatuses, industrial machinery and others.
Looking at several examples of MEMS element-carrying devices, for example, there is known the chip sealing type semiconductor device shown in FIG. 1 (see, Japanese Unexamined Patent Publication (Kokai) No. 8-184608). In this semiconductor device, a semiconductor substrate 151 has formed on it an IC chip 152 and has further formed on it a broad width metal bridge (Au ribbon) 108 covering the same. The metal bridge 108 is melt bonded to the semiconductor substrate 151 through bonding lands (Al) 155. Note that when forming the metal bridge 108, attention must be paid so that it does not contact bonding wires 153 and others in a cavity 109. The metal bridge 108 is sealed by an epoxy resin 110, then coated with a conductive paste 111. By configuring the device in this way, it is possible to secure a protective function by the metal bridge 108 and epoxy resin 110 and further to achieve a noise-seal effect by the conductive paste 111. The IC chip 152 on the semiconductor substrate 151 may also be a micromachine, for example, a rotor part of a micromotor.
Further, as shown in FIG. 2, there is known a surface mounting type chip scale package invented for eliminating the inconveniences of wire bonding (see, Japanese Unexamined Patent Publication (Kokai) No. 2002-43463). In this package, an element-supporting first substrate (silicon substrate etc.) 206 has an element active region 207 on which various types of MEMS elements are formed. Further, the element-supporting first substrate 206, as illustrated, has a cover-use second substrate (silicon, stainless steel, Coval™, copper, etc.) 212 filled with glass or ceramic 213 bonded to it by a conductive bonding material layer 210. The conductive bonding material layer 210 is formed from, for example, solder, gold, an anisotropic conductive sheet, a conductive epoxy resin, etc. Therefore, the cover-use second substrate 212 can be bonded by for example soldering, sheet bonding, epoxy bonding, anoding bonding, etc. Upon such bonding, a cavity 208 is formed. Further, during this bonding, element electrodes and bottom electrodes 211 each formed by Au, Al etc. are bonded so as to be conductive by the conductive bonding material layer 210. External interconnect electrodes 214 formed by Au, Al, etc. are electrically connected with the device electrodes 209 by the cover-use second substrate 212.
In the prior art, as explained above, when placing an MEMS element or other functional elements on the substrate of a package and the like to produce an electronic device, generally use is made of the method of fixing the functional element on the substrate by an adhesive, bonding sheet, etc., then using gold wires or other bonding wires for connecting the wirings, finally securing a lid by an adhesive (for example, epoxy-based adhesive), soldering (for example, silver paste), or other bonding means. Usually, the bonding means is coated on the four corners of the lid, then the lid is attached to the substrate and heated to secure it. With this method of securing the lid, however, the following numerous problems remain to be solved.
(1) Since the wire bonding system is used, the size of the package becomes larger.
(2) Since an adhesive is used as a bonding means, heat treatment at a raised temperature is required for curing. Deterioration of the element due to heat and adverse effects on the element characteristics due to the decomposed gas derived from the adhesive at the time of heating occur. Such adverse effects due to heating are important problems in electronic devices having mounted thereon functional elements provided with drive components and the like such as MEMS elements.
(3) When using a drive component, for example, a drive IC, in combination with an MEMS element, a package carrying the drive component etc. is necessary in addition to the package carrying the MEMS element, so a complicated structure, a drop in yield, a rise in production cost, etc. are unavoidable.
(4) With the method of resin sealing or use of a metal bridge, it is not possible to obtain a completely air-tight seal of the device.